The pathophysiology of fetal growth retardation is being investigated using a chronic sheep preparation. Fetal growth retardation is produced by a chronic administration of non-radioactive microspheres to the uterine artery supplying the pregnant horn to embolize the utero placental vascular bed. This procedure produces growth retardation of the fetal lamb, and the alteration in fetal organ weights is similar to fetal growth retardation seen in human pregnancies with utero placental insufficiency. Daily or weekly measurements of maternal and fetal blood gases, pH, blood glucose and oxygen contents are performed by use of indwelling catheters. The fetal circulation is studied with radioactive microspheres to determine the alteration in cardiac output and its distribution. Nucleic acid and protein content, and synthesis rates are measured in various fetal organs. Placental transfer and fetal consumption of glucose and oxygen are measured weekly or near the end of the experiment to define the relative roles of these metabolites. Preliminary investigations of therapy designed to positively alter the retarded fetal growth are being started.